The present disclosure relates to thermal power measurement and in particular but not exclusively to thermal power measurement for control of internal combustion engines.
Internal combustion engines are heat engines that perform the conversion of heat to mechanical energy. Internal combustion engines are very commonly used to provide mechanical energy: it is believed that there at least 1 billion internal combustion engines in the world. A large variety of internal combustion engine types exist, these types can include piston-type engines such as those used to provide mechanical energy to propel automobiles such as cars, trucks, motorcycles, busses, light aircraft, lawnmowers and the like. Other types internal combustion engine include types that do not have pistons, such as turbines and jet engines. Internal combustion engines can produce power in the range of a few watts to hundreds of megawatts.
In an internal combustion engine, a prepared fuel combusts with an oxidiser (often air but other sources of oxygen can be used) with the result that chemical energy stored in the fuel is transferred to the movement of an engine output (typically causing rotation of some form of drive shaft). Piston-based internal combustion engines can be produced that operate according to one of a number of different operational cycles. Among the most well-known reciprocating piston type internal combustion engine cycles are the Otto, Diesel, Brayton, Atkinson and Miller cycles. Rotary “piston” internal combustion engines can also exist and the among most well-known types is the Wankel engine (which follows approximately the Otto cycle). Turbines and jet engines (which can include turbofans, turbojets and rocket engines) are typically continuous rather than cyclical in operation.
Internal combustion engines can operate using a variety of different fuels. Examples include: petroleum oil, autogas, petrol, diesel, methane, kerosene, coal, biodiesel and hydrogen.
The performance of an internal combustion engine varies based upon factors such as energy efficiency, power to weight ratio, torque curve etc. With respect to energy efficiency, this affects the rate of recovery of usable energy from the combustion product's thermal energy. The energy can be transformed into work by utilising the increase in temperature and pressure created during combustion. The theoretical efficiency can be calculated by using an idealised thermodynamic cycle, for example, the Carnot cycle. In the Carnot cycle the efficiency depends only from high and low operating temperatures of the engine. At the present time, the maximal thermodynamic limit of efficiency for a typical internal combustion engine is understood to be about 40% for commercial Diesel engines, although laboratory tests are believed to have reached up to 49% efficiency (noting also that Otto cycle engines typically achieve thermodynamic lower efficiency than combined (Seiliger, or Trinkler, or Sabathe) cycle engines). The actual efficiency of a real internal combustion engine would be expected to be lower than such a theoretical maximum due to the impact of the real engine not benefiting from the idealised assumptions in the calculation.
Internal combustion engines produce as by-products of the combustion process air pollution emissions. These pollutants typically include CO, CO2, NOx and others. The level of pollutants produced by a given internal combustion engine will vary depending upon the fuel and combustion approach used, but also from operating parameters of the engine operating relating to the combustion process. Such operating parameters can include fuel/air ratio, operating temperature, fuel quality, etc. The typical control approach for an internal combustion engine is to monitor inputs and outputs and to vary the inputs in an effort to optimise the outputs.